What VR do I need to regulate 12V fans from 12v to ~7V?

Some more details and fewer question marks would not be amiss.

I assume by VR you mean variable resistor, in which case to get 7v you want to set up the equivalent of a voltage divider (google for schematic and equations if you don't know it) with the variable resistor and the fan, by assuming the resistance of the fan is the fan's rated voltage divided by current. This is an imperfect model actually, because the fans resistance is variable with speed and torque, but it will probably suffice.

Also, having a variable resistor with too much resistance is better than too little, because you can just not turn the variable resistor to it's full resistance if it does end up being too much.
 
just use the +12v to +5v trick. Don't know if it will biff your psu though. Why waste so much energy with the resistors or a voltage regulator.
 
I suppose a variable resistor or variable voltage regulator solution will give you a bit more flexibility, but I have never found myself wanting more flexbility than 12v, 7v and off using the 7v trick.

In case readers don't know, to easily power a fan at 7v, connect the 12v input to 12v as always, but connect the 0v, or ground, input to 5v. Usually one uses a SPDT switch to provide 12v, 7v and off configurations. Some power supplies don't like this very much, it can raise the voltage of the 5v rail, but I've never been able to find any problems from it.
 
I want it adjustable from 12V to 7V because I want it to run at about 9V :). Now, what resistor do i need? And how many watts should it be rated for?
 
iddqd said:
I want it adjustable from 12V to 7V because I want it to run at about 9V :). Now, what resistor do i need? And how many watts should it be rated for?
Click to expand...


What fan is it? What are it's current and power ratings? It's hard to give straight answers without knowing all the details. When you get that here, we'll go over the equations that you'll need to know to understand what values to use.

Honestly, I could just tell you a number, and then have it light on fire inside your case. Better to understand what's going on than to blindly trust what people tell you on the internet...
 
You should be able to figure out the resistor value for yourself based on my first post.

As fat-tony said, my first post implied, and this post states implicitly, you should really think about putting some thought and detail into your posts. Especially the first post in a thread you start.

As for wattage, you will need a resistor rated for V^2/R watts, where V is the voltage drop of the resistor you end up using, and R is the resistance.
 
They are 3x Sunon 40x40x20 "V1" maglev fans wired in parallel. Each one draws 1.2W.
 
1.2W means 12V at .1 amps. now use the formula posted above.... it's really not too difficult.
 
You'll never learn if we do your work for you. Plus, we're just as lazy as you are, so follow the instructions posted and you'll do fine. If you have any questions about any of the steps you need to take to calculate your resistor we'll be glad to answer them.

We'll even check your work after you do it, but I, personally, won't do it for you.
 
Voltage across a resitor, V = I * R,
You know what you want V to be (5V), you have I (.1A / fan, 3 fans, .3 A total)

Power dissipated by the resistor, P = I * V,
you will need a pot(entitmeter) rated to atleast that much power.

You want a linear taper pot. ('audio' are logarithmic, making small adjustments around the end of the turn is difficult).
 
You must log in or register to reply here.
Back
Top